1. Field of the Invention
The present invention relates to a scanning receiver. More specifically, the present invention relates to a receiver of a scanning type employing a digital frequency synthesizer employing a phase locked loop.
2. Description of the Prior Art
In the United States, for example, various kinds of information centering on the news have been broadcast on occasion using the broadcasting bands other than the general radio broadcasting bands. Such special purpose broadcasting is referred to as public service broadcasting, or P.S.B., and the carrier wave frequencies in the channels for such broadcasting are 30 through 50 MHz (low VHF band), 150 through 70 MHz (high VHF band), and 450 through 470 MHz (UHF band), which use frequency modulation signals. The public service broadcasting bands have been utilized for fire service, public security, highway patrol, weather forecast and the like. In such broadcasting, the program is not always broadcast but rather is broadcast upon occasion. Broadcasting frequencies have been allotted to the broadcasting channels normally at intervals of about 25 KHz.
In order to receive the frequency of a desired channel in such broadcasting, a scanning receiver is utilized. A typical prior art scanning receiver comprises a local oscillator circuit structured such that a desired crystal resonator can be selectively replaced. A desired one of a plurality of crystal resonators each allotted to a channel is sequentially selected automatically or manually in the local oscillator circuit by grounding the desired one of the crystal resonators to complete a circuit connection, whereby channel selection is achieved and a desired broadcasting frequency is received.
Meanwhile, a frequency scanning receiver of a frequency synthesizer type has been developed and put into practical use wherein a phase locked loop is employed as a local oscillator circuit.
A frequency synthesizer employing a phase locked loop usually comprises a voltage controlled oscillator the oscillation frequency of which is controllable as a function of an output voltage, as low pass filtered, obtainable from a phase detector, which is adapted to compare the phase or the frequency of the output from a reference oscillator and the phase or the frequency of an output from a programmable frequency divider adapted to frequency divide the output frequency from the said voltage controlled oscillator at the frequency division rate which is adatped to be variable as a function of a control signal. Automatic scanning of the oscillation frequency of the output from the said voltage controlled oscillator is effected by varying the said control signal and thus the frequency division rate of the programmable frequency divider. Therefore, if such a voltage controlled ocillator is used as a local oscillator of a tuner of a radio receiver, automatic scanning of the receiving frequency can be effected by varying the frequency division rate of the programmable frequency devider, as described above. Typically, the data concerning the frequency division rate of the programmable frequency divider, the receiving frequency band, and the like is stored in advance for each channel by presetting the data in a channel selection memory. Although it is desired to display the data stored in the channel selection memory by the use of a digital display, such an approach would be extremely expensive.
In general, a typical scanning receiver of a frequency synthesizer type often uses light emitting devices for the purpose of indication of a received channel. Therefore, skillful use of such light emitting devices to provide a visual indication of the data stored in the channel selection memory might enable an inexpensive implementation of a display for the data stored in the channel station selection memory.
Such a scanning receiver is usually structured such that an undesired channel is automatically skipped in the scanning operation, in order to disregard such an undesired channel so as to disable reception thereof. However, the conventional skip scanning scheme has been applied to a scanning receiver comprising a plurality of crystal resonators, as described above, wherein a crystal resonator corresponding to an undesired channel is prevented from being grounded by means of a channel skip instructing switch, whereby the said crystal resonator is prevented from being coupled to the local oscillator. However, according to the said scheme, in case where channel selection is effected by a scanning operation, with a squelch circuit disabled or turned off, particularly in a weak signal situation, a discordant noise is heard by the speaker each time the channel as set as not receivable is selected during the scanning operation i.e. each time the channel as set as not receivable is reached in the scanning operation.
Apparently, the above described scheme cannot be applied to a frequency scanning receiver of a frequency synthesizer type. Instead, it could be thought that a channel skip instructing switch is provided so as to disable the operation of a voltage controlled oscillator constituting a phase locked loop if and when such a channel is selected in the scanning operation. Nevertheless, if once the operation of the voltage controlled oscillator is disabled, it takes a time period until the voltage controlled oscillator starts again to operate to provide a predetermined oscillation frequency, such time period being generally referred to as a lock up time. Hence, it is necessary to select the time period required for shifting of one channel to be longer than the lock up time. As a result, such scheme has shortcomings that in selection of the channel by an automatic scanning operation, particularly in case of selection of an increased number of channels, much longer time is required for the scanning operation.
A conventional scanning receiver capable of receiving a plurality of bands comprises a band switching circuit, as disclosed in U.S. Pat. No. 3,665,318, for example, wherein the same number of switching devices such as switching transistors as the number of bands are provided. The band switching circuit is structured such that a manual band switching means provided at the input of each of the switching transistors is selectively set to a predetermined band for each channel and the input terminal of the switching transistor of the predetermined band is connected to a scanning signal oscillator for sequentially grounding the crystal oscillators, whereby the high frequency amplifier and mixer of the respective band is selectively energized through conduction of corresponding switching transistor. A different band switching circuit has also been disclosed in the United States patent application, Ser. No. 566,831, filed Apr. 10, 1975 now U.S. Pat. No. 4,000,470 and assigned to the same assignee as that of the present application, wherein band switching is effected by inserting a desired crystal oscillator into a corresponding receptacle out of a plurality of sets of crystal oscillator receptacles each provided for a channel by employing the same number of switching transistors as the number of bands. Nevertheless, if any of the above described prior art band switching circuits is employed in a frequency scanning receiver of a frequency synthesizer type, the circuit configuration becomes extremely complicated.